Ferritic stainless steel sheet excellent in press formability and workability and method for production thereof

ABSTRACT

A ferritic stainless steel sheet excellent in press formability and operability, characterized by: containing appropriate amounts of C, N, Cr, Si, Mn, P, S, Al, Ti and V, with the balance consisting of Fe and unavoidable impurities; having a solid lubricating film or films on one or both of the surfaces; and having a ratio Z, defined as Z=Z 1 /Z 2 , of less than 0.5, a tensile strength of 450 MPa or less and an average r-value of 1.7 or more, wherein Z 1  is a friction coefficient of the surface of a solid lubricating film and Z 2  that of the surface of a reference material coated with neither a coating nor lubricating oil. In the ferritic stainless steel sheet, the amounts of Sol-Ti and Insol-V may be regulated to appropriate ranges, wherein Sol-Ti means the amount of Ti existing in the state of solid solution in steel and Insol-V means the amount of V existing in the state of precipitation in steel.

TECHNICAL FIELD

[0001] The present invention relates to a ferritic stainless steel sheetexcellent in press formability and, particularly, deep drawability andshape fixability, and operability, and a method for producing the steelsheet.

BACKGROUND ART

[0002] A ferritic stainless steel sheet is used mostly in the form ofpress-formed members for kitchen facilities, electric appliances and thelike. However, as a ferritic stainless steel sheet is significantlyinferior in formability to a SUS304 steel sheet, a typical austeniticstainless steel sheet, it is prone to problems such as cracking in pressforming work.

[0003] In addition, compared with an ultra-low-carbon steel sheet, aferritic stainless steel sheet has problems of cracking during pressforming caused by its inferior deep drawability and poor shapefixability caused by its higher hardness.

[0004] Though there has been strong a need for replacing members ofaustenitic stainless steel, from the viewpoint of material costs, andmembers of ultra-low-carbon steel, from the viewpoint of corrosionprotection and appearance, with members of ferritic stainless steel, inthe event of employing ferritic stainless steel, the poor pressformability thereof has been a major obstacle.

[0005] In order to solve the problem, various methods have been studiedto improve the formability of a ferritic stainless steel sheet and amethod wherein the contents of C and N are lowered and elements such asTi and Nb are added is publicly known as one solution. Despite suchimprovements, however, formability of a ferritic stainless steel enoughto replace a member of austenitic stainless steel or ultra-low-carbonsteel with a member of ferritic stainless steel has not been realized.

[0006] It is a normal practice in the forming of a stainless steel sheetto apply lubricating oil such as machine oil to the sheet for preventingcracking and die galling during press forming. However, as this requiresa cleaning process for removing the lubricating oil after the pressforming, there has been a problem of poor operability. In addition,though it is desirable for lubricating oil to have a high viscosity forimproving formability, there has been a problem in that the higher theviscosity is, the more often oil remains after cleaning.

[0007] As stated above, the problems related to the formability of aferritic stainless steel sheet have not been solved satisfactorily and,even when the forming thereof is possible, the application and removalof lubricating oil have been required and thus considerable operabilityhas been sacrificed.

[0008] As is shown on page 254 of the Press Forming Handbook—In View ofForming Difficulty, the Second Edition, edited by the Thin Steel SheetForming Technology Workshop, a lubricated steel sheet having been coatedwith solid lubricating films beforehand and not requiring lubricatingoil has been developed recently. However, simply applying solidlubricating films to a presently existing ferritic stainless steel sheethas been insufficient to realize good formability comparable to that ofan austenitic stainless or ultra-low-carbon steel sheet.

[0009] As examples of the latest technologies, stainless steel sheetswherein the elongation after fracture and the Lankford values(hereinafter referred to as an r-value) thereof are raised, acrylic orurethane resin is applied to the surfaces, and by so doing improvedmaterial properties and the function of lubricating films are combinedtogether, are disclosed in Japanese Unexamined Patent Publication Nos.2002-60972 and 2002-60973.

[0010] By such a method, however, though a limiting drawing ratio(hereinafter referred to as an LDR) measured through cylindrical cupdeep-drawing test has been improved, formability has been insufficientfor applying the method to those components that require not only deepdrawability but also punch stretchability. In addition, by the method,spring back occurs after press forming and thus there has been a problemwith shape fixability as well.

SUMMARY OF THE INVENTION

[0011] The object of the present invention is, in view of the abovesituation, to provide: a ferritic stainless steel sheet excellent inpress formability to the extent of being able to replace an austeniticstainless or ultra-low-carbon steel sheet, capable of eliminating oilingand degreasing accompanying press forming, and thus excellent inoperability as well; and a method for producing the steel sheet.

[0012] The present invention includes a ferritic stainless steel sheetwherein: the upper limit of tensile strength is regulated and an averager-value is improved for suppressing the deterioration of the shapefixability of the steel sheet; and the state of precipitation and solidsolution in steel is optimized and solid lubricating films are appliedto the surfaces for securing a very excellent deep drawability. Thepresent invention has been established on the basis of identifying theproduction conditions of such a ferritic stainless steel sheet.

[0013] The present inventors, using various ferritic stainless steels,examined the formability of ferritic stainless steel sheets in the casesof changing chemical compositions, r-values and the states ofprecipitation and solid solution and further applying solid lubricatingfilms having different properties. An r-value and a tensile strengthwere measured through tensile tests in conformity with JIS (JapaneseIndustrial Standards) Z 2254 and Z 2241, respectively. An amount ofprecipitates was determined by quantitatively analyzing theelectrolytically extracted residues of a steel. An amount of asolid-solute element was determined by subtracting the relevantprecipitate amount obtained as above from the total addition amount ofthe relevant element.

[0014] Formability was evaluated through a cylindrical cup deep-drawingtest for measuring deep drawability, an Erichsen test for measuringpunch stretchability, a rectangular cup forming test for measuring bothdeep drawability and punch stretchability, and a hat-shape bending testfor measuring shape fixability.

[0015] The Erichsen test was carried out in conformity with JIS Z 2247.The results of the cylindrical cup deep-drawing test were evaluated inconformity with the TZP test specified in pages 468-469 of the PressForming Handbook—In View of Forming Difficulty, the Second Edition,edited by the Thin Steel Sheet Forming Technology Workshop.

[0016] In the rectangular cup forming test, a specimen was subjected todeep drawing by using a punch and a die both having a rectangularsection, and the formability was evaluated in terms of the drawing depthat the time when the specimen cracked.

[0017] The hat-shape bending test was carried out in conformity with thetest method specified in page 482 of the Press Forming Handbook—In Viewof Forming Difficulty, the Second Edition, and shape fixability wasevaluated in terms of the deviation of the angle of the portion bent bythe punch shoulder from a right angle.

[0018] A friction coefficient was evaluated through Bowden test. TheBowden test is a point contact type friction test that makes use ofreciprocating sliding of a steel ball and a steel sheet as described inpages 66-67 of the Plastic Forming Work Technology Series No. 3, ProcessTribology—Lubrication in Metal Forming edited by the Japan Society forTechnology of Plasticity.

[0019] As a result of the tests and examinations, it has been clarifiedthat a ferritic stainless steel sheet has formability equal to or betterthan that of a steel sheet of austenitic stainless steel SUS304 orTi-added ultra-low-carbon steel when some of the following conditions(A) to (F) are combined:

[0020] (A) The amount of P as a steel component is controlled to 0.02%or less.

[0021] (B) An average r-value is controlled to 1.7 or more.

[0022] (C) A tensile strength is controlled to 450 MPa or less.

[0023] (D) V is added by 0.1% or so, and the amount of V precipitatingin the form of carbonitride and the like is controlled to 0.01% or lessor, in other words, a certain amount of V existing in the state of solidsolution is secured.

[0024] (E) The amount of Ti existing in the form of solid solution in asteel is controlled to 0.16% or less.

[0025] (F) A solid lubricating film that as has a friction coefficientless than 50% of that of a reference material is applied. Morespecifically, the ratio Z₁/Z₂ is less than 0.5, wherein Z₁ is thefriction coefficient of a steel sheet coated with a solid lubricatingfilm and Z₂ the friction coefficient of a reference material that is asteel sheet having a surface roughness Ra in the range from 0.05 to 0.07μm and being coated with neither a solid lubricating film norlubricating oil.

[0026] The gist of the present invention, which has been established onthe basis of the above finding, is as follows:

[0027] (1) A ferritic stainless steel sheet excellent in pressformability and operability, characterized by: containing, in mass,

[0028] C: 0.001 to 0.01%,

[0029] N: 0.001 to 0.015%,

[0030] Cr: 10 to 19%,

[0031] Si: 0.01 to 0.8%,

[0032] Mn: 0.01 to 0.5%,

[0033] P: 0.01 to 0.02%,

[0034] S: less than 0.01%,

[0035] Al: 0.005 to 0.1%,

[0036] Ti: 0.05 to 0.25%, and

[0037] V: 0.03 to 0.12%,

[0038] with the balance consisting of Fe and unavoidable impurities;having a solid lubricating film or films on one or both of the surfaces;and having a ratio Z, defined as Z=Z₁/Z₂, of less than 0.5, a tensilestrength of 450 MPa or less and an average r-value of 1.7 or more,wherein Z₁ is a friction coefficient of the surface of a solidlubricating film and Z₂ that of the surface of a reference materialhaving a surface roughness Ra in the range from 0.05 to 0.07 μm andbeing coated with neither a coating nor lubricating oil.

[0039] (2) A ferritic stainless steel sheet excellent in pressformability and operability, characterized by: containing, in mass,

[0040] C: 0.001 to 0.01%,

[0041] N: 0.001 to 0.015%,

[0042] Cr: 10 to 19%,

[0043] Si: 0.01 to 0.8%,

[0044] Mn: 0.01 to 0.5%,

[0045] P: 0.01 to 0.02%,

[0046] S: less than 0.01%,

[0047] Al: 0.005 to 0.1%,

[0048] Ti: 0.05 to 0.25%,

[0049] Sol-Ti: 0.03 to 0.16%,

[0050] V: 0.03 to 0.12%, and

[0051] Insol-V: less than 0.01%,

[0052] with the balance consisting of Fe and unavoidable impurities,wherein Sol-Ti means the amount of Ti existing in the state of solidsolution in steel and Insol-V means the amount of V existing in thestate of precipitation in steel; having a solid lubricating film orfilms on one or both of the surfaces; and having a ratio Z, defined asZ=Z₁/Z₂, of less than 0.5, wherein Z₁ is a friction coefficient of thesurface of a solid lubricating film and Z₂ that of the surface of areference material having a surface roughness Ra in the range from 0.05to 0.07 μm and being coated with neither a coating nor lubricating oil.

[0053] (3) A ferritic stainless steel sheet excellent in pressformability and operability according to the item (2), characterized byhaving a tensile strength of 450 MPa or less and an average r-value of1.7 or more.

[0054] (4) A ferritic stainless steel sheet excellent in pressformability and operability according to any one of the items (1) to(3), characterized by containing, in mass, 0.0001 to 0.01% Mg.

[0055] (5) A ferritic stainless steel sheet excellent in pressformability and operability according to any one of the items (1) to(4), characterized by containing, in mass, 0.0005 to 0.005% B.

[0056] (6) A ferritic stainless steel sheet excellent in pressformability and operability according to any one of the items (1) to(5), characterized by containing, in mass, 0.1 to 3% Mo.

[0057] (7) A member of an electric appliance characterized by being madeof a ferritic stainless steel sheet excellent in press formability andoperability according to any one of the items (1) to (6).

[0058] (8) A method for producing a ferritic stainless steel sheetexcellent in press formability and operability according to any one ofthe items (1) and (4) to (6), characterized by: heating a ferriticstainless steel slab, containing, in mass,

[0059] C: 0.001 to 0.01%,

[0060] N: 0.001 to 0.015%,

[0061] Cr: 10 to 19%,

[0062] Si: 0.01 to 0.8%,

[0063] Mn: 0.01 to 0.5%,

[0064] P: 0.01 to 0.02%,

[0065] S: less than 0.01%,

[0066] Al: 0.005 to 0.1%,

[0067] Ti: 0.05 to 0.25%,

[0068] V: 0.03 to 0.12%, and,

[0069] as required, one or more of

[0070] Mg: 0.0001 to 0.01%,

[0071] B: 0.0005 to 0.005%, and

[0072] Mo: 0.1 to 3%,

[0073] with the balance consisting of Fe and unavoidable impurities, toa temperature in the range from 1,050° C. to 1,250° C.; subjecting theheated slab to hot rolling at a total reduction ratio of 95% or more, afinish rolling temperature of 750° C. to 950° C. and a coilingtemperature of 500° C. to 800° C.; then, after annealing the hot-rolledsteel sheet or without annealing it, subjecting it to cold rolling at atotal reduction ratio of 60 to 95%; heating the cold-rolled steel sheetto a temperature in the range from 800° C. to 950° C.; holding it at thetemperature for 0 to 30 sec.; thereafter cooling it; and then coating itwith a solid lubricant.

[0074] (9) A method for producing a ferritic stainless steel sheetexcellent in press formability and operability according to any one ofthe items (2) and (4) to (6), characterized by: heating a ferriticstainless steel slab, containing, in mass,

[0075] C: 0.001 to 0.01%,

[0076] N: 0.001 to 0.015%,

[0077] Cr: 10 to 19%,

[0078] Si: 0.01 to 0.8%,

[0079] Mn: 0.01 to 0.5%,

[0080] P: 0.01 to 0.02%,

[0081] S: less than 0.01%,

[0082] Al: 0.005 to 0.1%,

[0083] Ti: 0.05 to 0.25%,

[0084] V: 0.03 to 0.12%, and,

[0085] as required, one or more of

[0086] Mg: 0.0001 to 0.01%,

[0087] B: 0.0005 to 0.005%, and

[0088] Mo: 0.1 to 3%,

[0089] with the balance consisting of Fe and unavoidable impurities, toa temperature in the range from 1,050° C. to 1,250° C.; subjecting theheated slab to hot rolling at a finish rolling temperature of 750° C. to950° C. and a coiling temperature of 500° C. to 800° C.; then, afterannealing the hot-rolled steel sheet or without annealing it, subjectingit to cold rolling; heating the cold-rolled steel sheet to a temperaturein the range from 800° C. to 950° C.; holding it at the temperature for0 to 30 sec.; thereafter cooling it to 500° C. or lower at a coolingrate of 10° C./sec. or more; and then coating it with a solid lubricant.

[0090] (10) A method for producing a ferritic stainless steel sheetexcellent in press formability and operability according to any one ofthe items (3) to (6), characterized by: heating a ferritic stainlesssteel slab, containing, in mass,

[0091] C: 0.001 to 0.01%,

[0092] N: 001 to 0.015%,

[0093] Cr: 10 to 19%,

[0094] Si: 0.01 to 0.8%,

[0095] Mn: 0.01 to 0.5%,

[0096] P: 0.01 to 0.02%,

[0097] S: less than 0.01%,

[0098] Al: 0.005 to 0.1%,

[0099] Ti: 0.05 to 0.25%,

[0100] V: 0.03 to 0.12%, and,

[0101] as required, one or more of

[0102] Mg: 0.0001 to 0.01%,

[0103] B: 0.0005 to 0.005%, and

[0104] Mo: 0.1 to 3%,

[0105] with the balance consisting of Fe and unavoidable impurities, toa temperature in the range from 1,050° C. to 1,250° C.; subjecting theheated slab to hot rolling at a total reduction ratio of 95% or more, afinish rolling temperature of 750° C. to 950° C. and a coilingtemperature of 500° C. to 800° C.; then, after annealing the hot-rolledsteel sheet or without annealing it, subjecting it to cold rolling at atotal reduction ratio of 60 to 95%; heating the cold-rolled steel sheetto a temperature in the range from 800° C. to 950° C.; holding it at thetemperature for 0 to 30 sec.; thereafter cooling it to 500° C. or lowerat a cooling rate of 10° C./sec. or more; and then coating it with asolid lubricant.

[0106] (11) A method for producing a ferritic stainless steel sheetexcellent in press formability and operability according to any one ofthe items (8) to (10), characterized by subjecting the cold-rolled steelsheet to temper rolling at a reduction ratio of 0.3 to 1.5% afterheating and cooling it but before coating it with a solid lubricant.

[0107] The Most Preferred Embodiment

[0108] The essence of the present invention is, on the premise that asolid lubricating film is applied to a steel sheet, to lower the tensilestrength of the steel sheet and increase the average r-value thereof forimproving the workability, especially the shape fixability, of the steelsheet, and also to optimize the state of precipitation and solidsolution in the steel by adequately controlling the steel components andproduction conditions for further improving the deep drawability of thesteel sheet. The present invention is explained hereafter in detail.

[0109] In the first place, the reasons are explained for restrictingsteel components in the present invention. Note that, in thedescriptions below, percentage figures are in mass.

[0110] C and N: When either C or N is added abundantly, formabilitydeteriorates and also the amount of Ti required for fixing themincreases. For those reasons, the upper limits of contents of C and Nare set at 0.01% and 0.015%, respectively. The lower limit is set at0.001% for both in consideration of steel refining costs.

[0111] Cr: Cr is an element necessary for securing corrosion resistance,which constitutes the most fundamental property of stainless steel. WhenCr is added by 10% or more, corrosion resistance significantly improves.For this reason, the lower limit of a Cr content is set at 10%. When Cris added by more than 19%, however, formability deteriorates. For thisreason, the upper limit of a Cr content is set at 19%.

[0112] Si: Si is an element used as a deoxidizing agent. When an Sicontent is more than 0.8%, formability significantly deteriorates. Forthis reason, the upper limit of an Si content is set at 0.8%. Si isunavoidably included in steel by 0.01% and, for this reason, the lowerlimit of an Si content is set at 0.01% in consideration of steelrefining costs.

[0113] Mn: When Mn is added abundantly, formability deteriorates. Forthis reason, the upper limit of an Mn content is set at 0.5%. The lowerlimit of an Mn content is set at 0.01% in consideration of steelrefining costs.

[0114] P: P is a particularly important constituent element for thepresent invention. In the case where a solid lubricating film isapplied, formability significantly improves by controlling the contentof P to 0.02% or less. For this reason, the upper limit of a P contentis set at 0.02%. When a P content is less than 0.01%, however, steelrefining costs increase significantly. For this reason, the lower limitof a P content is set at 0.01%.

[0115] P is included in raw materials such as ferrochromium and, as aresult, a 10-19% Cr steel usually contains P by 0.02 to 0.03%.Therefore, it is necessary to employ an intensive dephosphorizationtreatment or adequately select raw materials to satisfy the above upperlimit.

[0116] S: When S is added abundantly, corrosion resistance deteriorates.For this reason, an S content is limited to less than 0.01%.

[0117] Al: Al is used as a deoxidizing agent, but a large additionamount of Al deteriorates formability. For this reason, the upper limitof an Al content is set at 0.1%. The lower limit of an Al content is setat 0.005% as the least required amount for deoxidizing.

[0118] Ti: Ti is an element that combines with C, N, etc. to formprecipitates and, by so doing, improves formability. Since it isnecessary to add Ti by 0.05% or more for obtaining the formabilityimprovement effect, the lower limit of a Ti content is set at 0.05%.When Ti is added by more than 0.25%, formability may deteriorate, insome cases, on the contrary. For this reason, the upper limit of a Ticontent is set at 0.25%.

[0119] V: V is another particularly important constituent element forthe present invention. The lower limit of a V content is set at 0.03% asthe least amount required for obtaining a formability improvement effectin the case where a solid lubricating film is applied. When V is addedby more than 0.12%, however, no further formability improvement effectis obtained and, what is worse, raw material costs increase. For thosereasons, the upper limit of a V content is set at 0.12%.

[0120] V is included in ferrochromium raw material and, as a result, Vis unavoidably included in steel by about 0.02% in some cases. Since theV coming from raw material has the same effect as the V addeddeliberately, it is necessary to control the total V content to therange specified above.

[0121] In a steel sheet according to any one of the aforementioned items(2) to (6), the amounts of Sol-Ti and Insol-V are regulated as explainedbelow for the purpose of significantly enhancing deep drawability.

[0122] Sol-Ti: With regard to Ti, it is further necessary to control theamount of Ti in the state of solid solution. Sol-Ti refers to the amountof Ti existing in the state of solid solution in a steel. When theamount of solid-solute Ti exceeds 0.16%, the formability of a steelsheet coated with a solid lubricating film deteriorates. For thisreason, the upper limit of an Sol-Ti amount is set at 0.16%. On theother hand, for inhibiting the intergranular corrosion of a weld, it isnecessary to secure a solid-solute Ti amount of 0.03% or more. For thisreason, the lower limit of a Sol-Ti amount is set at 0.03%. An amount ofsolid-solute Ti may be obtained by quantitatively analyzing theelectrolytically extracted residues of a steel, thus determining theamount of Ti existing in the form of precipitate, and subtracting the Tiprecipitate amount from the total addition amount of Ti.

[0123] Insol-V: In the case of V, it is necessary to regulate the amountof V precipitating as the precipitates of V. Insol-V refers to all Vamounts existing in the form of precipitates in a steel. When the amountof V precipitates is 0.01% or more, the formability of a steel sheetcoated with a solid lubricating film deteriorates. For this reason, theupper limit of an Insol-V amount is set at less than 0.01%. An amount ofV precipitates may be obtained by quantitatively analyzing theelectrolytically extracted residues of a steel.

[0124] Hereafter, the elements to be optionally added to a steelaccording to the present invention are explained.

[0125] Mg: Mg is an element that makes the structure of a weld fine andthus improves the formability of the weld. For this reason, Mg may beadded as an optional element when the forming of a weld is required. Theeffect of improving the formability of a weld shows up with an Mgaddition amount of 0.0001% or more. For this reason, the lower limit ofan Mg content is set at 0.0001%. The upper limit of an Mg content is setat 0.01% in consideration of raw material costs.

[0126] B: B is an element that improves workability at secondaryworking, and, therefore, B may be added in the case where a plurality offorming processes are applied. The effect of improving workability atsecondary working shows up with a B addition amount of 0.0005% or more.When B is added in excess of 0.005%, however, toughness may deterioratein some cases. For this reason, the upper limit of a B content is set at0.005%.

[0127] Mo: Mo is an element that improves corrosion resistance, and,therefore, Mo may be added in the case where a steel material issubjected to a severely corrosive environment. The effect of improvingcorrosion resistance shows up with an Mo addition amount of 0.1% ormore, and, therefore, the lower limit of an Mo content is set at 0.1%.When Mo is added in excess of 3%, on the other hand, raw material costsincrease significantly and, in addition, formability deteriorates. Forthose reasons, the upper limit of an Mo content is set at 3%.

[0128] In a steel sheet according to any one of the aforementioned items(1) and (3) to (6), the average r-value is determined to be 1.7 or moreand the tensile strength to be 450 MPa or less. By the combination ofthe two determinations, press formability superior to that of aconventional steel sheet is secured and shape fixability is improvedremarkably. When an average r-value is less than 1.7 or a tensilestrength is more than 450 MPa, spring back after press forming mayincrease and a good product shape may not be obtained stably, in somecases.

[0129] No upper limit of an average r-value is specified but the maximumvalue obtainable without incurring any significant cost increase inexisting production facilities is 3.0. No lower limit of a tensilestrength is specified either but the lowest tensile strength of astainless steel containing a large amount of Cr is usually 330 MPa. Anr-value may be measured in conformity with JIS Z 2254, and a tensilestrength in conformity with JIS Z 2241.

[0130] As stated before, formability constitutes a problem with aferritic stainless steel sheet more often than with an austeniticstainless steel sheet. In this connection, the friction coefficient of aferritic stainless steel sheet surface constitutes one of the factorsgoverning the formability of the steel sheet at press forming. Thisproblem has conventionally been coped with by applying oil as describedearlier. However, since the oil has to be removed afterward, theapplication of oil deteriorates operability.

[0131] In view of this, the present inventors focused attention on thepossibility that cleaning for removing oil might not be required if asteel sheet was pre-coated with a solid lubricating film capable ofsufficiently reducing the friction coefficient of a surface and usedwithout removing the film.

[0132] The feature to be fulfilled by a solid lubricating film is that aratio Z defined as Z=Z₁/Z₂ is limited to less than 0.5, wherein Z₁ isthe friction coefficient of the surface of a solid lubricating film andZ₂ is that of the surface of a reference material having a surfaceroughness Ra conditioned in the range from 0.05 to 0.07 μm and beingcoated with neither a coating nor lubricating oil.

[0133] That is, it is necessary to control a ratio Z to less than 0.5,or otherwise good formability is not realized. It is desirable that theratio Z is as small as possible. However, a ratio Z of 0.1 or less mayoften result in a cost disadvantage. For that reason, a ratio Z of about0.3 is said to be a desirable level from the viewpoint of the balancebetween formability and cost.

[0134] The reason why Z is defined as a ratio of a friction coefficientof the surface of a solid lubricating film to that of the surface of areference material in the present invention is that a frictioncoefficient measured by a test method such as Bowden test, wherein thesurface of a specimen contacts a tool, may fluctuate in accordance withenvironmental conditions (temperature, moisture, etc.) and theconditions of a test apparatus. Whereas the absolute value of a frictioncoefficient fluctuates in accordance with measurement conditions, arelative ratio of friction coefficients does not change significantly asfar as they are measured under the same conditions.

[0135] Therefore, the present inventors reasoned that the frictioncoefficient fluctuation caused by measurement conditions could beminimized if a friction coefficient of the surface of a solidlubricating film and that of the surface of a reference material havinga surface roughness Ra in the range from 0.05 to 0.07 μm and beingcoated with neither a coating nor lubricating oil were measured underthe same conditions and the ratio between the two friction coefficientswas used.

[0136] A friction coefficient can be measured by aforementioned Bowdentest for example. Otherwise, a friction coefficient may be obtained bytaking the steps of: pulling a specimen and measuring the pulling forcewhile pressing a tool onto the specimen under a prescribed load;repeating the above procedures twice or more changing the pressing load;plotting the pulling forces against the pressing loads; and defining thegradient of the curve thus plotted as the friction coefficient.

[0137] In the present invention, the value of Z is not affected by thecontact area of a tool and a specimen since it is defined as the ratiobetween the friction coefficients of a specimen and a referencematerial. Therefore, any tool is acceptable as long as the portion of atool contacting with a specimen has a spherical shape, and the materialand the size of a tool are not specified.

[0138] Note that a surface roughness Ra is an arithmetic averageroughness specified in JIS B 0601 as a parameter for expressing surfaceroughness. The repeatability of the measured values of the surfaceroughness Ra of a metal surface is far better than that of the measuredfriction coefficients.

[0139] Note also that it is necessary to limit the surface roughness ofa reference material in a narrow range because a friction coefficient issignificantly affected by a surface roughness.

[0140] Further, when a surface roughness is large, a measured frictioncoefficient fluctuates more. Therefore, the surface roughness Ra of areference material is limited in the range from 0.05 to 0.07 μm.

[0141] Furthermore, any material is acceptable as a reference materialas long as the material is a stainless steel sheet since the influenceof a material on a friction coefficient is insignificant. However, apreferable reference material is a ferritic stainless steel sheet andthe best reference material is a ferritic stainless steel sheet havingchemical components in the ranges specified in the present invention.

[0142] A solid lubricating film is defined as a film of a lubricant thatis in a solid state at room temperature. Either an organic or inorganicfilm may be used as a solid lubricating film as far as a value of Zsatisfies the aforementioned condition. Urethane, acrylic, olefin,polyester, epoxy resins and the like are counted as organic lubricants,and silicate, titanium oxide, phosphate, chromate, zirconate films andthe like are counted as inorganic lubricants.

[0143] In the case of an organic lubricant, a suitable film thickness isin the range from 0.5 to 10 μm and it is desirable to add wax such asthat of a fluoride or polyethylene system by 0.5 to 30% of the solidcontent of resin. In the case of an inorganic lubricant, a suitabledeposition amount is 10 to 500 mg/m².

[0144] A removable type coating film, which can be washed away bydegreasing, may be used as a solid lubricating film. However, a ferriticstainless steel sheet is sometimes used without paint coating and, insuch a case, a post-treatment such as degreasing or chemical treatmentis not required and therefore a non-removable type solid lubricatingfilm, which need not be removed from a final product, is suitable.

[0145] Further, when a ferritic stainless steel sheet is used for aproduct requiring compatibility with good appearance and design, it isdesirable to use a clear solid lubricating film. Furthermore, as thepresent invention makes it unnecessary to apply a meticulous surfacefinishing for the purpose of reducing a friction coefficient, asubstantial cost reduction is expected for some applications along withimprovement in operability.

[0146] Any method may be employed for applying a solid lubricating filmin the present invention. For example, brush coating or spray coatingmay be employed, or otherwise, roll coating, curtain coating or thelike, which is widely used for applying an organic film, may also beemployed. Since an important issue of the present invention is thefriction coefficient of the surface of a solid lubricating film, dueconsideration must be paid not only to a coating method but also to amethod of drying and baking.

[0147] In order for a solid lubricating film according to the presentinvention to have additional functions such as corrosion resistance,stain resistance, compatibility with good appearance and design, and thelike, in combination, an anticorrosive pigment, a metal powder, and thelike, may be added. In such a case too, the friction coefficient of thesurface of a solid lubricating film must satisfy the requirement of thepresent invention. In that sense, a multiple-layered coating film havingthe outermost layer satisfying the requirement of the present inventionmay also be used.

[0148] A ferritic stainless steel sheet according to the presentinvention is produced through the processes of melting, casting, hotrolling, cold rolling and annealing and, thereafter, is coated with asolid lubricating film. The steel sheet may be subjected to anotherannealing process after the hot rolling. When annealing is applied to ahot-rolled steel sheet, it is desirable to use a continuous annealingline for the annealing in consideration of production efficiency. Theannealing of a hot-rolled steel sheet may be carried out under normalconditions and no specific conditions are regulated in the presentinvention.

[0149] Further, from the viewpoint of securing good surface properties,a steel sheet may be subjected to annealing during the course of coldrolling. In this case, the annealing may be carried out under normalconditions since the annealing does not adversely affect formability.Furthermore, it is desirable to subject a hot-rolled steel sheet topickling. In this case, normal conditions may be applied for picklingliquor, processing time and other operation parameters. After coldrolling and annealing, a steel sheet may be subjected to temper rolling.

[0150] When a heating temperature in a hot rolling process is lower than1,050° C., the re-solution of precipitates does not occur sufficientlyin a slab. However, when it is higher than 1,250° C., crystal grainscoarsen and hot workability deteriorates. For those reasons, a heatingtemperature in a hot rolling process must be in the range from 1,050° C.to 1,250° C.

[0151] For further suppressing the coarsening of crystal grains, themost suitable upper limit of the heating temperature is 1,200° C. It ispreferable to measure a heating temperature by attaching thermocouplesto a slab. Here, when a slab is held in a reheating furnace for one houror longer, the temperature of furnace atmosphere may be regarded as theheating temperature of the slab.

[0152] When a finish rolling temperature is lower than 750° C., rollingloads increase, and cracks and surface defects are likely to occur to ahot-rolled steel sheet. When a finish rolling temperature exceeds 950°C., on the other hand, work strain imposed during hot rolling isrelieved and recrystallization hardly occurs in a coiling process or anannealing process after hot rolling. Therefore, a finish rollingtemperature must be in the range from 750° C. to 950° C.

[0153] When a coiling temperature in a hot rolling process is lower than500° C., the state of precipitates may change and formability maydeteriorate in some cases. When it is higher than 800° C., on the otherhand, dense oxides form on the surfaces of a steel sheet and a burden inthe subsequent pickling process increases. For those reasons, a coilingtemperature in a hot rolling process must be in the range from 500° C.to 800° C.

[0154] A finish rolling temperature and a coiling temperature of hotrolling can be measured with a radiation thermometer. It is preferableto calibrate a thermometer beforehand by measuring emissivity atdifferent radiation temperatures. A correct emissivity value can beobtained by: attaching a thermocouple to the surface of a stainlesssteel sheet; heating the steel sheet and thereafter measuring thetemperature change during cooling using the thermocouple and a radiationthermometer; and repeating the above procedures twice or more whileadjusting the emissivity of the radiation thermometer.

[0155] In the final annealing process after cold rolling, it isnecessary to heat a cold-rolled steel sheet to 800° C. to 950° C. for 0to 30 sec. When a heating temperature in the final annealing process islower than 800° C., un-recrystallized crystas may remain, crystal grainsmay be made fine, and thus the workability of a product steel sheet maydeteriorate in some cases.

[0156] When a heating temperature in the final annealing process exceeds950° C., on the other hand, crystal grains coarsen and a rough surfaceappears after forming. The effect of annealing shows up as long as anannealing temperature reaches a temperature in the above specified rangeeven though a retention time at the annealing temperature is 0 sec.However, if a retention time exceeds 30 sec., crystal grains may coarsenin some cases. An annealing temperature and a retention time in thefinal annealing process can be adjusted by controlling the atmospherictemperature of an annealing furnace and a steel sheet traveling speed.

[0157] It is preferable to apply temper rolling after the finalannealing from the viewpoint of eliminating yield elongation, correctingthe shape of a steel sheet and so forth. A reduction ratio of less than0.3% at temper rolling may be insufficient from the viewpoint of theelimination of yield elongation and the correction of a steel sheetshape, but, on the other hand, a reduction ratio exceeding 1.5% causesthe hardening of a steel sheet and thus cracking occurs during formingand/or shape fixability deteriorates.

[0158] For the above reasons, it is preferable to regulate a reductionratio at temper rolling in the range from 0.3 to 1.5%. The most suitableupper limit of a reduction ratio at temper rolling for obtaining goodformability is less than 1.0%.

[0159] Note that a total reduction ratio at temper rolling is a value,expressed in terms of percentage, obtained by dividing the differencebetween the thickness of a cold-rolled steel sheet after finish coldrolling and the thickness of the cold-rolled steel sheet after temperrolling by the former thickness.

[0160] A solid lubricating film is applied to a cold-rolled steel sheetafter subjected to temper rolling or not subjected to temper rolling. Itis preferable to degrease the surface of a steel sheet before a solidlubricating film is applied. A preferable method of applying a solidlubricating film is to coat the solid lubricating film with brushcoating, spray coating, roll coating, curtain coating or the like, drythe film, and then bake it at 70° C. to 200° C. for 0 to 1,800 sec.

[0161] In order to produce a steel sheet having a low tensile strengthand a significantly improved shape fixability according to any one ofthe items (1) and (3) to (6), it is necessary to control the reductionratios in hot rolling and cold rolling processes in respectiveappropriate ranges as specified in the production method according tothe item (8) or (10).

[0162] When a total reduction ratio in a hot rolling process is lessthan 95%, a rolling texture may not develop and sufficient deepdrawability and shape fixability may not be obtained in some cases. Forthis reason, the lower limit of a total reduction ratio in a hot rollingprocess must be 95% or more.

[0163] The higher the lower limit of a total reduction ratio in a hotrolling process, the better. A preferable total reduction ratio in a hotrolling process is 97% or more in consideration of the relationshipbetween the thickness of a slab and a hot-rolled steel sheet and themost suitable total reduction ratio is 98% or more. No upper limit isspecified for a total reduction ratio in a hot rolling process, but themaximum reduction ratio is about 99.8% in presently availabletechnologies. Note that a total reduction ratio at hot rolling is avalue, expressed in terms of percentage, obtained by dividing thedifference between the thickness of a slab and the thickness of ahot-rolled steel sheet by the former thickness.

[0164] When a total reduction ratio at cold rolling is less than 60%, arolling texture does not develop sufficiently and formabilitydeteriorates as a result. When a total reduction ratio at cold rollingexceeds 95%, on the other hand, a rolling texture develops excessivelyand anisotropy increases as a result. For those reasons, a totalreduction ratio at cold rolling must be in the range from 60 to 95%. Apreferable range thereof is from 75 to 95%. Note that a total reductionratio at cold rolling is a value, expressed in terms of percentage,obtained by dividing the difference between the thickness of ahot-rolled steel sheet and the thickness of a cold-rolled steel sheetafter finish cold rolling by the former thickness.

[0165] In order to produce a steel sheet having the well-controlledstate of precipitation and solid solution and a significantly improveddeep drawability according to any one of the items (2) to (6), it isnecessary to control a cooling rate in the final annealing process aftera hot rolling or cold rolling process in an appropriate range asspecified in the production method according to the item (9) or (10).

[0166] In the case of a steel sheet according to any one of the items(2) to (6), the cooling rate of a steel sheet in the final annealingprocess is of particular importance for changing the state ofprecipitation and solid solution and improving deep drawability.

[0167] That is to say, it is necessary to cool a steel sheet to atemperature of 500° C. or lower at a cooling rate of 10° C./sec. or moreafter heating. When a cooling rate is lower than 10° C./sec.,workability may deteriorate in some cases. No upper limit is specifiedfor a cooling rate, but 100° C./sec. is enough.

[0168] The reason why the temperature range wherein a cooling rate isspecified is determined to be 500° C. or lower is that precipitationtends to occur in the temperature range from 500° C. to 950° C. No lowerlimit is particularly specified for a cooling end temperature, and asteel sheet may be cooled up to the room temperature at a cooling rateof 10° C./sec. or more. A cooling rate can be obtained by calculating acooling time from a steel sheet traveling speed and the length a coolingzone and then dividing the difference between the temperatures of asteel sheet at the entry and the exit of the cooling zone by theresulting cooling time.

[0169] It is preferable to use an air blower or the like for the coolingof a steel sheet. When water is used for cooling, sufficient drying isrequired and, moreover, impurities contained in water may remain on asteel sheet surface and cause an uneven coating film in some cases.

[0170] By regulating the conditions of the production processes asmentioned above in addition to the aforementioned steel components, aferritic stainless steel sheet excellent in press formability andoperability wherein the r-value, the tensile strength and the state ofprecipitation and solid solution in the steel are controlled and a solidlubricating film is applied is obtained.

[0171] A steel sheet produced by the above production method isexcellent in press formability and shape fixability, can be formed intocomplicated shapes, and can take the advantage of good appearance of alubricating film. Therefore, a steel sheet according to the presentinvention is suitable as a material for members of electric appliances.

[0172] Concrete examples of applicable members are: outer panels andinternal components of an electric keep-warm vessel, a microwave oven, arefrigerator, a washing machine, a dish washer and the like; and outerpanels of a TV set, a videotape recorder and the like. When a ferriticstainless steel sheet according to the present invention is used forthese applications, a preferable thickness range is from 0.4 to 1.5 mm.

EXAMPLE

[0173] Examples of the present invention are described below.

Example 1

[0174] The ferritic stainless steels shown in Table 1 were melted andsteel sheets 0.5 to 0.6 mm in thickness were produced from them throughthe process combination of hot rolling, annealing of hot-rolled steelsheets (some hot-rolled steel sheets were excluded), cold rolling, andannealing. In the annealing of the hot-rolled steel sheets, the heatingtemperatures were 800° C. to 950° C. and the retention times were 0 to30 sec. In the final annealing, the annealing temperatures were changedand the steel sheets were cooled by air with an air blower. In the finalannealing, the retention times were 10 sec. and the cooling endtemperatures were 500° C. or lower. All the steel sheets were subjectedto temper rolling at the reduction ratio of 0.5% after the finalannealing.

[0175] The heating temperatures (referred to as SRT), the finish rollingtemperatures (referred to as FT), the coiling temperatures (referred toas CT) and the total reduction ratios at the hot rolling, the totalreduction ratios at the cold rolling, and the annealing temperatures atthe final annealing are shown in Table 2. SUS304 steel sheets were alsoprepared as comparative steel sheets.

[0176] The r-value and the tensile strength of each of the steel sheetsthus produced were measured in the longitudinal, thickness and widthdirections and the average values were calculated respectively. Anr-value was measured in conformity with JIS Z 2254 and a tensilestrength was measured in conformity with JIS Z 2241.

[0177] The solid lubricating films of acrylic, acrylic-urethane, epoxy,epoxy-urethane, urethane-polyethylene and urethane systems were appliedto the steel sheets with a roll coater, dried and then baked at thetemperatures of 70° C. to 200° C. for 0 to 1,800 sec.

[0178] The friction coefficients of the steel sheets after applying thesolid lubricating films and the friction coefficient of the referencematerial having a surface roughness Ra of 0.06 μm and being not coatedwith a solid lubricating film were measured by Bowden test withoutapplying lubricating oil, and the ratios Z of the friction coefficientsof the steel sheets with the solid lubricating films to the frictioncoefficient of the reference material were calculated.

[0179] Formability was measured by the TZP test and the rectangular cupforming test, and the LDRs and the rectangular cup drawing depths wereused respectively as the indicators of formability. The TZP test wascarried out using the blanks 90 to 120 mm in diameter and a punch 50 mmin diameter. The rectangular cup forming test was carried out using arectangular cylindrical punch and a rectangular die, and the formabilitywas evaluated by the drawing depth at the time when a specimen cracked.

[0180] Shape fixability was evaluated by hat-shape bending test, whereinthe angle of the portion of a specimen bent by the shoulder of a punchwas measured and the shape fixability was evaluated by the splay angledefined by the deviation of the measured angle from a right angle.

[0181] The production conditions, the r-values, the tensile strengths,the ratios Z, the LDRs, the rectangular cup drawing depths and the splayangles of the steel sheets are shown also in Table 2.

[0182] The steel sheets according to the present invention showedformability equal to or better than that of the SUS304 steel sheet. Onthe other hand, in the cases of the steel sheet A produced at the totalhot rolling reduction ratio of 85% and the steel sheet E produced at thetotal hot rolling reduction ratio of 94%, those total hot rollingreduction ratios being lower than those in the range specified in thepresent invention, and the steel sheets B and C produced at the totalcold rolling reduction ratio of 50%, the total cold rolling reductionratio being lower than that in the range specified in the presentinvention, the r-values were lower than those in the range specified inthe present invention, the LDRs and the rectangular cup drawing depthsdecreased and the splaying angles increased.

[0183] Further, in the cases of the steel sheets A, D and E produced atthe final annealing temperature of 750° C. which was lower than atemperature in the range specified in the present invention, therecrystallization was insufficient, the tensile strength was high, and,as a result, the rectangular cup drawing depths decreased, and thesplaying angles increased and thus the shape fixability deteriorated.

[0184] Furthermore, in the cases of the steel sheets B and D having theratios Z equal to 0.7, as the properties of the solid lubricating filmswere insufficient, the rectangular cup drawing depths decreased. In thecase of the steel sheet F, as the amounts of P and Ti exceeded theamounts in the respective ranges specified in the present invention, thetensile strength was high and the rectangular cup drawing depth and theshape fixability decreased. TABLE 1 Steel Chemical components (in mass%) symbol C Si Mn P S Al Cr N Ti V Others Remarks A 0.0082 0.55 0.350.018 0.002 0.035 11.2 0.008 0.21 0.06 Invention sample B 0.0044 0.150.15 0.011 0.003 0.021 17.2 0.002 0.15 0.10 1.3 Mo Invention sample C0.0021 0.06 0.11 0.014 0.004 0.008 16.3 0.010 0.16 0.08 0.0008 Mg,Invention 0.0007 B sample D 0.0040 0.21 0.08 0.012 0.008 0.042 18.50.009 0.12 0.11 0.0022 Mg Invention sample E 0.0011 0.05 0.12 0.0110.003 0.011 14.6 0.007 0.12 0.09 0.5 Mo Invention sample F 0.0015 0.050.12 0.025 0.003 0.011 17.0 0.006 0.31 0.09 0.5 Mo Comparative sampleSUS304 0.0400 0.50 0.80 0.030 0.001 0.005 18.0 0.040 — — 8.0 NiComparative sample

[0185] TABLE 2 Final Hot rolling Annealing Cold rolling annealing Totalreduction of hot- Total reduction Annealing Tensile Steel SRT FT CTratio rolled ratio temperature r- strength symbol ° C. ° C. ° C. % sheet% ° C. value MPa A 1160 880 620 98 Not 80 750 1.6 472 applied 870 60098.1 Not 70 840 2.0 420 applied Applied 80 840 2.0 410 890 650 85Applied 65 860 1.6 431 B 1150 850 580 98 Not 50 900 1.4 410 applied Not70 880 2 420 applied Applied 80 885 2.2 405 Applied 85 920 2.1 415 C1150 790 750 98.1 Not 50 875 1.5 361 applied Not 90 900 2.4 378 appliedApplied 80 850 2.3 380 D 1180 780 640 98.2 Applied 85 900 2.2 415 Not 93750 1.8 530 applied Not 83 900 2.2 420 applied Applied 80 880 2 410 E1150 820 640 98 Applied 81 900 2.3 415 Not 80 750 1.8 460 applied Not 80890 1.9 410 applied 800 610 94 Applied 80 875 1.6 410 F 1150 820 64098.2 Applied 80 900 2.3 465 810 620 98 Not 80 890 2.1 475 applied SUS3041250 900 620 98.5 Applied 70 1050  0.9 595 Formability Rectangular cupSplaying Steel drawing depth angle symbol Z LDR mm ° Remarks A 0.3 1.948 4 Comparative sample 0.35 2.1 65 1 Invention sample 0.25 2.1 65 1Invention sample 0.3 1.8 48 3 Comparative sample B 0.3 1.7 59 3Comparative sampl 0.31 2.4 65 1 Invention sample 0.28 2.4 63 1 Inventionsample 0.7 1.8 58 1 Comparative sample C 0.3 1.7 59 3 Comparativ sample0.29 2.7 64 1 Invention sample 0.33 2.4 63 1 Invention sample D 0.7 1.951 1 Comparative sample 0.31 1.9 45 5 Comparative sample 0.32 2.3 61 1Invention sample 0.22 2.4 63 1 Invention sample E 0.19 2 63 1 Inventionsample 0.28 1.8 50 3 Comparative sample 0.3 2.1 61 1 Invention sample0.31 1.9 60 2 Comparative sample F 0.3 2 57 3 Comparative sample 0.3 1.852 4 Comparative sample SUS304 0.3 2 60 4 Comparative sample

Example 2

[0186] The ferritic stainless steel sheets 0.5 to 0.6 mm in thicknesswere produced in the same manner as in Example 1 except that theannealing temperatures of the final annealing were changed and thecooling rates at the cooling of the steel sheets were also changed bychanging the air flow amounts of an air blower used for the air coolingof the steel sheets.

[0187] The retention times of the annealing were 10 sec. and the coolingend temperatures were 500° C. or lower. The SRTS, the FTs, the CTs, thetotal reduction ratios at the hot rolling, the total reduction ratios atthe cold rolling, the annealing temperatures and cooling rates at thefinal annealing are shown in Table 3. SUS304 steel sheets were alsoprepared as comparative steel sheets.

[0188] The average r-values of the steel sheets thus produced weremeasured also in the same manner as in Example 1. The electrolyticallyextracted residues of the steel sheets were analyzed quantitatively andthe amounts of Sol-Ti and Insol-V were obtained from the analysisresults of respective components. The same solid lubricating films asused in Example 1 were applied to the surfaces of the steel sheets andthe ratios Z were determined through Bowden test. Thereafter, the LDRsand the rectangular cup drawing depths were also evaluated.

[0189] The r-values, the amounts of Sol-Ti and Insol-V, the ratios Z,the LDRs and the rectangular cup drawing depths are shown also in Table3.

[0190] The steel sheets according to the present invention showedformability equal to or better than that of the SUS304 steel sheet. Onthe other hand, in the case of the steel sheet A produced at the finalannealing temperature of 1,050° C. which was higher than a temperaturein the range specified in the present invention, the amount of Sol-Tiwas larger than that in the range specified in the present invention,the crystal grains coarsened, and, as a result, the LDR and therectangular cup drawing depth decreased.

[0191] Further, in the case of the steel sheet B produced at the finalannealing temperature of 780° C. which was lower than a temperature inthe range specified in the present invention, the recrystallization wasinsufficient, and, as a result, the LDR and the rectangular cup drawingdepth decreased.

[0192] Furthermore, in the cases of the steel sheets A, B and E producedat the final annealing cooling rate of 5° C./sec. which was lower than acooling rate in the range specified in the present invention and thesteel sheet C produced at the final annealing cooling rate of 2° C./sec.which was also lower than a cooling rate in the range specified in thepresent invention, the amounts of Insol-V were larger than an amount inthe range specified in the present invention, and, as a result, therectangular cup drawing depths decreased.

[0193] Still further, in the case of the steel sheet D having theratio'Z equal to 0.68, the properties of the solid lubricating film wereinsufficient and, as a consequence, the rectangular cup drawing depthdecreased.

[0194] Still further, in the case of the steel sheet F, since theamounts of P and Ti exceeded the amounts in the respective rangesspecified in the present invention, the amount of Sol-Ti was larger thanan amount in the range specified in the present invention and therectangular cup drawing depth decreased. TABLE 3 Hot rolling AnnealingCold rolling Final annealing Total reduction of hot- Total reductionAnnealing Cooling Steel SRT FT CT ratio rolled ratio temperature ratesymbol ° C. ° C. ° C. % sheet % ° C. ° C./sec. A 1160 870 600 98.1 Not70 840 15 applied Not 70 875 5 applied 800 620 98 Applied 80 880 15Applied 80 1050 15 B 1150 850 580 98 Not 70 880 20 applied Not 75 875 5applied Applied 81 860 15 Applied 85 780 18 C 1150 790 750 98.1 Not 90877 20 applied Applied 90 900 2 Applied 90 875 30 D 1180 780 650 98.2Not 83 860 15 applied Not 80 875 15 applied Applied 70 880 40 E 1150 820640 98 Not 80 875 20 applied Not 85 895 5 applied Applied 80 890 30 F1150 820 640 98.2 Applied 80 900 15 810 620 98 Not 80 890 15 appliedSUS304 1250 900 620 98.5 Applied 70 1050  Formability Rectangular cupSteel Sol-Ti Insol-V drawing depth symbol mass % mass % Z LDR mm RemarksA 0.15 <0.01 0.35 2.1 65 Invention sample 0.15 0.02 0.29 1.9 65Comparative sample 0.15 <0.01 0.31 2.1 65 Invention sample 0.18 <0.010.35 1.9 59 Comparative sample B 0.13 <0.01 0.29 2.4 62 Invention sample0.12 0.02 0.31 2.1 59 Comparative sample 0.13 <0.01 0.31 2.4 62Invention sample 0.12 <0.01 0.31 1.9 45 Comparative sample C 0.12 <0.010.29 2.7 64 Invention sample 0.11 0.02 0.29 1.9 58 Comparative sample0.12 <0.01 0.31 2.8 64 Invention sample D 0.07 <0.01 0.32 2.3 61Invention sample 0.06 <0.01 0.25 2.4 61 Invention sample 0.08 <0.01 0.681.9 58 Comparative sample E 0.10 <0.01 0.3 2.1 61 Invention sample 0.090.02 0.29 1.9 59 Comparative sample 0.10 <0.01 0.22 2.3 63 Inventionsample F 0.24 <0.01 0.3 2 53 Comparative sample 0.25 <0.01 0.31 1.8 52Comparative sample SUS304 — — 0.3 2 60 Comparative sample

Example 3

[0195] The ferritic stainless steel sheets 0.5 to 0.6 mm in thicknesswere produced in the same manner as in Example 1 except that theannealing temperatures of the final annealing were changed and thecooling rates at the cooling of the steel sheets were also changed bychanging the air flow amounts of an air blower used for the air coolingof the steel sheets.

[0196] The retention times of the annealing were 10 sec. and the coolingend temperatures were 500° C. or lower. The SRTS, the FTs, the CTs, thetotal reduction ratios at the hot rolling, the total reduction ratios atthe cold rolling, the annealing temperatures and cooling rates at thefinal annealing are shown in Table 4. SUS304 steel sheets were alsoprepared as comparative steel sheets.

[0197] The r-values and the tensile strengths of the steel sheets thusproduced were measured also in the same manner as in Example 1 and theamounts of Sol-Ti and Insol-V were obtained in the same manner as inExample 2. The same solid lubricating films as used in Examples 1 and 2were applied to the surfaces of the steel sheets, the ratios Z weredetermined through Bowden test, and the formability tests were carriedout. The r-values, the amounts of Sol-Ti and Insol-V, the ratios Z, theLDRs, the rectangular cup drawing depths and the splaying angles areshown also in Table 4.

[0198] The steel sheets according to the present invention showedformability equal to or better than that of the SUS304 steel sheet. Onthe other hand, in the case of the steel sheet A produced at the finalannealing temperature of 1,050° C. which was higher than a temperaturein the range specified in the present invention, the amount of Sol-Tiwas larger than an amount in the range specified in the presentinvention, the crystal grains coarsened, and, as a result, the LDR andthe rectangular cup drawing depth decreased.

[0199] Further, in the case of the steel sheet B produced at the finalannealing temperature of 780° C. which was lower than a temperature inthe range specified in the present invention, the recrystallization wasinsufficient, the tensile strength was high and, as a result, therectangular cup drawing depth decreased, and the splay angle increasedand thus shape fixability deteriorated.

[0200] Furthermore, in the cases of the steel sheets A, B and E producedat the final annealing cooling rate of 5° C./sec. which was lower than acooling rate in the range specified in the present invention and thesteel sheet C produced at the final annealing cooling rate of 2° C./sec.which was also lower than a cooling rate in the range specified in thepresent invention, the amounts of Insol-V were larger than an amount inthe range specified in the present invention, and, as a result, the LDRsand the rectangular cup drawing depths decreased.

[0201] Still further, in the case of the steel sheet D having the ratioZ equal to 0.68, the properties of the solid lubricating film wereinsufficient and, as a consequence, the rectangular cup drawing depthdecreased. In the case of the steel sheet F, as the amounts of P and Tiexceeded the amounts in the respective ranges specified in the presentinvention, the tensile strength was high and the rectangular cup drawingdepth and the shape fixability decreased. TABLE 4 Cold Hot rollingAnnealing rolling Final annealing Total reduction of hot- Totalreduction Annealing Cooling Tensile Steel SRT FT CT ratio rolled ratiotemperature rate r- strength symbol ° C. ° C. ° C. % sheet % ° C. °C./sec. value MPa A 1160 870 600 98 Not 72 845 20 1.9 410 applied Not 72880 5 1.9 409 applied 880 620 98.1 Applied 85 885 18 1.9 400 Applied 851050 17 1.9 395 B 1150 850 580 98.1 Not 75 875 19 2 410 applied Not 80870 5 2 402 applied Applied 80 850 16 2.2 421 Applied 85 780 20 2 451 C1150 790 750 98 Not 85 880 18 2.4 381 applied Applied 85 890 2 2 391Applied 85 870 20 2.4 368 D 1180 780 640 98 Not 85 850 16 2.2 410applied Not 90 880 18 2.2 442 applied Applied 75 885 30 2.2 406 E 1150820 640 98.1 Not 85 880 15 1.9 405 applied Not 88 900 5 1.9 420 appliedApplied 72 895 25 2.1 433 F 1150 820 640 98.1 Applied 72 890 20 2.3 480810 620 98 Not 72 880 18 2.1 470 applied SUS304 1250 900 620 98.2Applied 80 1050 0.9 610 Formability Rectangular cup Splaying SteelSol-Ti Insol-V drawing depth angle symbol mass % mass % Z LDR mm °Remarks A 0.15 <0.01 0.35 2.1 65 1 Invention sample 0.14 0.02 0.29 1.758 1 Comparative sample 0.15 <0.01 0.31 2.1 65 1 Invention sample 0.19<0.01 0.35 1.9 59 1 Comparative sample B 0.12 <0.01 0.29 2.4 62 1Invention sample 0.13 0.02 0.31 1.9 59 1 Comparative sample 0.12 <0.010.31 2.4 62 1 Invention sample 0.12 <0.01 0.31 2 45 3 Comparative sampleC 0.11 <0.01 0.29 2.7 64 1 Invention sample 0.13 0.02 0.29 1.9 58 1Comparative sample 0.11 <0.01 0.31 2.7 64 1 Invention sample D 0.07<0.01 0.32 2.3 61 1 Invention sample 0.06 <0.01 0.31 2.3 61 1 Inventionsample 0.08 <0.01 0.61 1.9 58 1 Comparative sample E 0.09 <0.01 0.3 2.161 1 Invention sample 0.12 0.02 0.29 1.9 59 1 Comparative sample 0.09<0.01 0.22 2.3 63 1 Invention sample F 0.29 <0.01 0.3 2 54 4 Comparativesample 0.29 <0.01 0.31 1.8 53 3 Comparative sample SUS304 — — 0.3 2 60 4Comparative sample

INDUSTRIAL APPLICABILITY

[0202] The present invention makes it possible to provide a ferriticstainless steel sheet excellent in press formability and operability anda method for producing the steel sheet, and thus to contribute toexpanding the range of the use of ferritic stainless steel.

[0203] Therefore, the industrial significance of the present inventionis extremely large.

1. A ferritic stainless steel sheet excellent in press formability andoperability, characterized by: containing, in mass, C: 0.001 to 0.01%,N: 0.001 to 0.015%, Cr: 10 to 19%, Si: 0.01 to 0.8%, Mn: 0.01 to 0.5%,P: 0.01 to 0.02%, S: less than 0.01%, Al: 0.005 to 0.1%, Ti: 0.05 to0.25%, and V: 0.03 to 0.12%, with the balance consisting of Fe andunavoidable impurities; having a solid lubricating film or films on oneor both of the surfaces; and having a ratio Z, defined as Z=Z₁/Z₂, ofless than 0.5, a tensile strength of 450 MPa or less and an averager-value of 1.7 or more, wherein Z₁ is a friction coefficient of thesurface of a solid lubricating film and Z₂ that of the surface of areference material having a surface roughness Ra in the range from 0.05to 0.07 μm and being coated with neither a coating nor lubricating oil.2. A ferritic stainless steel sheet excellent in press formability andoperability, characterized by: containing, in mass, C: 0.001 to 0.01%,N: 0.001 to 0.015%, Cr: 10 to 19%, Si: 0.01 to 0.8%, Mn: 0.01 to 0.5%,P: 0.01 to 0.02%, S: less than 0.01%, Al: 0.005 to 0.1%, Ti: 0.05 to0.25%, Sol-Ti: 0.03 to 0.16%, V: 0.03 to 0.12%, and Insol-V: less than0.01%, with the balance consisting of Fe and unavoidable impurities,wherein Sol-Ti means the amount of Ti existing in the state of solidsolution in steel and Insol-V means the amount of V existing in thestate of precipitation in steel; having a solid lubricating film orfilms on one or both of the surfaces; and having a ratio Z, defined asZ=Z₁/Z₂, of less than 0.5, wherein Z₁ is a friction coefficient of thesurface of a solid lubricating film and Z₂ that of the surface of areference material having a surface roughness Ra in the range from 0.05to 0.07 μm and being coated with neither a coating nor lubricating oil.3. A ferritic stainless steel sheet excellent in press formability andoperability according to claim 2, characterized by having a tensilestrength of 450 MPa or less and an average r-value of 1.7 or more.
 4. Aferritic stainless steel sheet excellent in press formability andoperability according to any one of claims 1 to 3, characterized bycontaining, in mass, 0.0001 to 0.01% Mg.
 5. A ferritic stainless steelsheet excellent in press formability and operability according to anyone of claims 1 to 4, characterized by containing, in mass, 0.0005 to0.005% B.
 6. A ferritic stainless steel sheet excellent in pressformability and operability according to any one of claims 1 to 5,characterized by containing, in mass, 0.1 to 3% Mo.
 7. A member of anelectric appliance characterized by being made of a ferritic stainlesssteel sheet excellent in press formability and operability according toany one of claims 1 to
 6. 8. A method for producing a ferritic stainlesssteel sheet excellent in press formability and operability according toany one of claims 1 and 4 to 6, characterized by: heating a ferriticstainless steel slab, containing, in mass, C: 0.001 to 0.01%, N: 0.001to 0.015%, Cr: 10 to 19%, Si: 0.01 to 0.8%, Mn: 0.01 to 0.5%, P: 0.01 to0.02%, S: less than 0.01%, Al: 0.005 to 0.1%, Ti: 0.05 to 0.25%, V: 0.03to 0.12%, and, as required, one or more of Mg: 0.0001 to 0.01%, B:0.0005 to 0.005%, and Mo: 0.1 to 3%, with the balance consisting of Feand unavoidable impurities, to a temperature in the range from 1,050° C.to 1,250° C.; subjecting the heated slab to hot rolling at a totalreduction ratio of 95% or more, a finish rolling temperature of 750° C.to 950° C. and a coiling temperature of 500° C. to 800° C.; then, afterannealing the hot-rolled steel sheet or without annealing it, subjectingit to cold rolling at a total reduction ratio of 60 to 95%; heating thecold-rolled steel sheet to a temperature in the range from 800° C. to950° C.; holding it at the temperature for 0 to 30 sec.; thereaftercooling it; and then coating it with a solid lubricant.
 9. A method forproducing a ferritic stainless steel sheet excellent in pressformability and operability according to any one of claims 2 and 4 to 6,characterized by: heating a ferritic stainless steel slab, containing,in mass, C: 0.001 to 0.01%, N: 0.001 to 0.015%, Cr: 10 to 19%, Si: 0.01to 0.8%, Mn: 0.01 to 0.5%, P: 0.01 to 0.02%, S: less than 0.01%, Al:0.005 to 0.1%, Ti: 0.05 to 0.25%, V: 0.03 to 0.12%, and, as required,one or more of Mg: 0.0001 to 0.01%, B: 0.0005 to 0.005%, and Mo: 0.1 to3%, with the balance consisting of Fe and unavoidable impurities, to atemperature in the range from 1,050° C. to 1,250° C.; subjecting theheated slab to hot rolling at a finish rolling temperature of 750° C. to950° C. and a coiling temperature of 500° C. to 800° C.; then, afterannealing the hot-rolled steel sheet or without annealing it, subjectingit to cold rolling; heating the cold-rolled steel sheet to a temperaturein the range from 800° C. to 950° C.; holding it at the temperature for0 to 30 sec.; thereafter cooling it to 500° C. or lower at a coolingrate of 10° C./sec. or more; and then coating it with a solid lubricant.10. A method for producing a ferritic stainless steel sheet excellent inpress formability and operability according to any one of claims 3 to 6,characterized by: heating a ferritic stainless steel slab, containing,in mass, C: 0.001 to 0.01%, N: 001 to 0.015%, Cr: 10 to 19%, Si: 0.01 to0.8%, Mn: 0.01 to 0.5%, P: 0.01 to 0.02%, S: less than 0.01%, Al: 0.005to 0.1%, Ti: 0.05 to 0.25%, V: 0.03 to 0.12%, and, as required, one ormore of Mg: 0.0001 to 0.01%, B: 0.0005 to 0.005%, and Mo: 0.1 to 3%,with the balance consisting of Fe and unavoidable impurities, to atemperature in the range from 1,050° C. to 1,250° C.; subjecting theheated slab to hot rolling at a total reduction ratio of 95% or more, afinish rolling temperature of 750° C. to 950° C. and a coilingtemperature of 500° C. to 800° C.; then, after annealing the hot-rolledsteel sheet or without annealing it, subjecting it to cold rolling at atotal reduction ratio of 60 to 95%; heating the cold-rolled steel sheetto a temperature in the range from 800° C. to 950° C.; holding it at thetemperature for 0 to 30 sec.; thereafter cooling it to 500° C. or lowerat a cooling rate of 10° C./sec. or more; and then coating it with asolid lubricant.
 11. A method for producing a ferritic stainless steelsheet excellent in press formability and operability according to anyone of claims 8 to 10, characterized by subjecting the cold-rolled steelsheet to temper rolling at a reduction ratio of 0.3 to 1.5% afterheating and cooling it but before coating it with a solid lubricant.